Binders or binder systems for foundry cores and molds are well known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resultant mixture is rammed, blown, or otherwise formed to the desired shape or pattern, and then cured with the use of catalyst and/or heat to a solid, cured state.
Resin binders used in the production of foundry molds and cores are often cured at high temperatures to achieve the fast-curing cycles required in foundries. However, in recent years, resin binders have been developed which cure at a low temperature, to avoid the need for high-temperature curing operations which have higher energy requirements and which often result in the production of undesirable fumes.
One group of processes which do not require heating in order to achieve curing of the resin binder are referred to as "cold-box" processes. In such processes, the binder components are coated on the aggregate material, such as sand, and the material is blown into a box of the desired shape. Curing of the binder is carried out by passing a gaseous catalyst at ambient temperatures through the molded resin-coated material. In such processes, the binder components normally comprise a polyhydroxy component and a polyisocyanate component. These cure to form a polyurethane in the presence of a gaseous amine catalyst.
Another group of binder systems which do not require gassing or heating in order to bring about curing are known as "no-bake" systems. These "no-bake" systems also frequently employ an aggregate material, such as sand coated with a polyhydroxy component and a polyisocyanate component. In this case, the coated sand is usually mixed with a liquid tertiary amine catalyst just before the sand is placed into a holding pattern or core box, and the material is allowed to cure at ambient temperatures or slightly higher.
Binder systems which employ polyhydroxy and polyisocyanate components usually contain solvents as well. These provide mixtures of lower viscosity which are more readily coated on sand. Although the solvents do not enter to any significant degree into the reaction between the polyisocyanate and the polyhydroxy compound, they can affect the reaction. As a result, cores and molds prepared using different solvents for the binders may have quite different properties.
A number of U.S. patents related to polyurethane binder systems are directed to specific solvent additives. Such patents include U.S. Pat. Nos. 3,476,696; 3,905,934; 3,985,699; 4,231,914; and 4,273,179.
Although binder systems containing these solvents have been quite useful, such systems with urethane binders still exhibit certain disadvantages. For example, there are certain applications where a refractory coating on the cores or molds is required to obtain the necessary surface finish on castings produced from the cores or molds. Water-based refractory coatings are preferred for this purpose. However, when cores and molds, prepared using urethane binders, are washed with water-based refractory coating material, they exhibit an undesired loss of tensile strength.
Now it has been found, in accordance with this invention, that the use of certain solvents, not hitherto used with urethane binder systems, overcomes this deficiency and produces cores and molds with good strength even after they have been washed with water-based core washes.